Conventionally, as a drop-on-demand type ink jet wherein ink droplets are discharged from a nozzle connected to a pressure generating chamber which is filled with ink by producing pressure wave (acoustic wave) therein using an electromechanical transducer such as a piezoelectric actuator, examples described in Japanese Patent Publication No. SHO53-12138 and Japanese Patent Application Laid-Open No. HEI10-193587 have been generally known.
FIG. 11 is a diagram showing an example of a constitution of a recording head in the ink jet recording apparatus disclosed in the above publications. A pressure generating chamber 111 is connected with an ink supply channel 114 for conducting ink from an ink tank, which is not illustrated, via a nozzle 112 for ejecting ink and a common ink chamber 113.
Besides, the pressure generating chamber 111 is provided with a diaphragm 115 on the bottom surface thereof. On the occasion of discharging an ink droplet, pressure wave is produced in the pressure generating chamber 111 by displacing the diaphragm 115 using a piezoelectric actuator 116 installed outside of the pressure generating chamber 111 so that a volume change occurs in the pressure generating chamber 111. By the pressure wave, the ink filling the pressure generating chamber 111 is partially discharged out via the nozzle 112, and flies as an ink droplet 117. The flying ink droplet 117 lands on a recording medium such as recording paper and forms a recording dot. Characters and images are recorded on the recording paper by repeatedly executing such formation of the recording dot based on image data.
Various shapes of driving waveforms are applied to the piezoelectric actuator 116 corresponding to the sizes of the ink droplets to be ejected, however, in the case of discharging a large-diameter ink droplet used for recording characters or a dense part, the driving waveform as shown in FIG. 12(a) is adopted in general.
That is, in voltage changing process 121, the ink droplet is discharged by increasing voltage applied to the piezoelectric actuator 116 and so rapidly decreasing the volume of the pressure generating chamber 111, and after that, the voltage is returned to standard voltage (Vb) in voltage changing process 122.
Incidentally, a relationship between driving voltage and the operation of the piezoelectric actuator 116 varies depending on the constitution of the piezoelectric actuator 116 and/or polarization direction. In the present invention, it is assumed that when the driving voltage is increased, the volume of the pressure generating chamber 111 decreases, and contrary, when the driving voltage is decreased, the volume thereof increases.
In addition, a driving waveform as shown in FIG. 12(b) may be adopted in order to stabilize the flying condition of the ink droplet. In the waveform, voltage changing process 123′ for slightly increasing the volume of the pressure generating chamber 111 is added just before voltage changing process 121′ for ejecting the ink droplet, and the ejecting state of the ink droplet is stabilized by the operation of the added voltage changing process 123′. Namely, meniscus at an aperture of the nozzle is retracted to the side of the pressure generating chamber 111 by slightly expanding the pressure generating chamber 111 before ejection, and thereby the form of the meniscus just before the ejection becomes slightly concave.
When the ejection of the ink droplet is executed on such condition where the meniscus is in concave form, it is possible to reduce the influence of wet on the nozzle surface or nonuniform shape of the aperture of the nozzle (burr, etc.), and thus stabilize the ejecting direction of the ink droplet and occurrence condition of a satellite.
FIG. 13(a) shows a flying condition of an ink droplet on the occasion of ejecting the ink droplet by the driving waveform of FIG. 12(a). There is a tail 132 at the back of the ink droplet ejected from a nozzle aperture 131. The tail separates from a main drop 133 during the flying process, and forms a satellite 134. The satellite 134 becomes a spherical shape in the flying process, flying at a speed equal to or a little slower than the speed of the main drop, and reaches the recording paper.
FIG. 14(a) is a model diagram showing the condition of the meniscus just after ejecting a large ink droplet. After ejecting the ink droplet, a concave-shaped meniscus 142 is formed since the quantity of ink in a nozzle 141 decreases. The concave-shaped meniscus 142 gradually returns up to the aperture portion of the nozzle by the operation of surface tension of the ink, and recovers the condition before the ejection. Such recovery action of the meniscus is called “refill”.
In the case of discharging ink droplets in succession, unless the following ejection is executed after the refill has been completed, the diameter and speed of the ink droplet will be destabilized and the steady successive ejection cannot be performed. That is, the maximum driving frequency of the ink jet recording head is subject to the speed of the refill. Accordingly, in the conventional ink jet recording head, the head has been designed to speed up the refill so that the recording speed (driving frequency) accelerates as fast as it can.
Concretely, the widths of the nozzle and the supply channel, the length and sectional area of the pressure generating chamber and so forth are designed so as to lessen fluid channel resistance (acoustic resistance) and inertance (inertia) in the ink fluid channel between the ink tank and nozzle.
However, with the ink jet recording head of these days improving in picture quality and speed, there have arisen the following problems which the conventional design of a driving waveform and a head is unable to cope with.
The first problem is that a satellite generated on the ejection of a large-diameter ink droplet (big drop) deteriorates picture quality. As described above, the satellite is generated when the big drop is discharged. If there was a wide gap between landing positions of the main drop and satellite, the picture quality will be notably deteriorated. Particularly, in the case where the diameter of the ink droplet is modulated in grades (drop diameter modulation) for printing out a gradation image such as a photograph in high quality, it is impossible to obtain the high-quality picture without controlling the landing position of the satellite precisely.
The deterioration in picture quality due to the error of the landing position of the satellite as above is remarkable especially when the environment temperature changes. FIGS. 13(b) and 13(c) are model diagrams showing changes in a flying condition due to the environment temperatures. In the case where a big drop was ejected adopting the driving waveform of the conventional example shown in FIG. 12(a), a normal flying condition as shown in FIG. 13(a) was obtained in a room temperature environment (25□) and a high temperature environment (40□), and there was no problem in a recording result.
However, when the recording was executed in a low temperature environment (5□), the tail of the ink droplet became extremely long as shown in FIG. 13(c), and it was observed that a low velocity satellite 136 was produced. Such low velocity satellite 136 lands onto recording paper in a floating condition, causing great deterioration in the sharpness of a whole image. In addition, the satellite stains a blank space of the image, and thereby picture quality is notably deteriorated.
Moreover, in the case where another conventional driving waveform was employed, while a normal flying condition as shown in FIG. 13(a) was obtained in the room temperature environment and low temperature environment, it was observed that a large number of fine particulate satellites 135 as shown in FIG. 13(b) were produced in the high temperature environment. Such fine particle satellites 135 easily stick onto the surface of a nozzle plate, causing deterioration in the ejecting direction of the drops during the successive ejection and an ejection failure.
As described above, in order to realize a good image recording at any time regardless of a change in the environment temperature, the satellite generated during the ejection of a big drop should be always maintained in a normal condition. However, there has been no established control method of the satellite, and therefore it has been very difficult to keep a satellite in good condition constantly in a wide range of temperature.
The second problem which the conventional design of a driving waveform and a head cannot cope with is the acceleration of the refill. As mentioned above, the speed of the refill needs to be accelerated for raising an ejection frequency of an ink droplet. For the sake of that, it is necessary to widen the nozzle, ink supply channel, a sectional area of the pressure generating chamber and the like to reduce the fluid resistance and inertance in the ink fluid channel. However, an increase in the diameter of the nozzle is a disadvantage in ejecting a fine ink droplet which is essential for recording a high quality picture, and therefore the diameter of the nozzle cannot be made wider than a certain width (about 35 μm is the upper limit in general).
Moreover, since a gain in the diameter of the ink supply channel causes deterioration in the efficiency of the ejection, it is also difficult to make it wider drastically. With regard to the pressure generating chamber, it is an advantage in accelerating the refill to widen the sectional area and shorten the length, however, since the shape of the pressure generating chamber has a close relationship with a resonance frequency of pressure wave and the density of the nozzles in rows, etc., there is little freedom of the shape, and it is difficult to gain the refill velocity drastically by a change in the shape of the pressure generating chamber.
Namely, in the conventional ink jet recording head, there has been a problem that it is difficult to increase the refill velocity to a large extent by improving a constitution of the head, and thus it is impossible to sufficiently meet a recent demand for improving the recording speed.
It is therefore an object of the present invention, which has been devised to solve the problems, to provide a driving method of an ink jet recording head and an ink jet recording apparatus suitable for a high frequency driving, which are capable of accelerating refill velocity after the ejection of a big ink droplet as well as recording image with high quality at any time regardless of a change in environment temperature by flying a satellite on ejecting the big drop in good condition constantly.